Field of the Invention
The present invention relates to a DRR image generation method and particularly relates to the DRR image generation method and a DRR image generation apparatus that are applied to compare an X-ray radiograph that is radiographed by the X-ray radiography system comprising an X-ray tube and an X-ray detector when a radiation therapy is performed by irradiating a therapeutic beam to a subject.
Description of the Related Art
DRR (Digital Reconstructed Radiography) image that is a virtual fluoroscopic projection utilizing 3-dimensional image collected by an X-ray CT (computed or computer tomography) apparatus can be e.g., applied to positioning of a patient as the subject. Specifically, with regard to the therapy apparatus that performs the radiation therapy by irradiating the therapeutic beam to the patient, DRR image is applied to calculate a gap of the X-ray radiograph (image) when the positioning of the patient is completed. In addition, since the positioning of the patient is completed, a medical doctor and others make sure whether the positional gap between the DRR image and the X-ray radiograph is in an acceptable range or not by reading out such images.
When such DRR image is generated, a geometry that is a geometrical arrangement between the X-ray tube and the X-ray detector, which are sandwiching the CT image, is virtually reconstructed on the computer. And a line integral relative to a CT data voxel value on the line connecting between the virtual X-ray tube and the virtual X-ray detector is obtained. And a line integral of the CT value is converted to the line integral of a linear attenuation coefficient to calculate the X-ray attenuation, and then a relative X-ray dose that should reach to each pixel is calculated based on such attenuation. And a pixel value of each pixel is calculated to obtain the DRR image by executing normalization as to the relative X-ray dose (Patent Document 1).